I am not trying to be a scare monger but I would keep a close eye on that,
i could be wrong and it maybe nothing, but from what i can see it looks suspiciously like something internal. I lost a rescue axolotl when it pushed its bowels and intestines out due to malnutrition, others have lost them to impaction.If it has been on gravel at anytime it could have swallowed some and this could be the problem.
Tell her to make sure there is nothing in the tank that can be swallowed other than food and this will limit the risk of it pushing anything out that it shouldnt.
I hope all remains well and i am sorry if i have caused needless worry but i can only comment on what i can see.
Hopefully someone else will come up with a differant opinion and her axolotl stays healthy a long time